MYRTLE BEACH, South Carolina — A tightened field vying for the Republican nomination took aim at frontrunner Mitt Romney in a fiery debate here just days before South Carolina’s presidential primary.

Before a raucous audience late Monday, the four candidates lagging behind Romney in polls ahead of Saturday’s key vote slammed the former Massachusetts governor and traded charges over a rash of nasty attack ads by outside groups.

Former House Speaker Newt Gingrich, who is trailing in the battle to be the party’s standard-bearer in the November election against President Barack Obama, came out swinging against Romney, scoring several hits with the crowd.

Gingrich said Romney should answer questions about his time as a venture capitalist with a company called Bain Capital, after Romney’s rivals slammed him for buying out firms to leave them bankrupt and reap the rewards.

The multi-millionaire Romney was also called on to release his tax records, and said he may do so later in the year, without making a firm commitment.

Romney hit back against the attacks, led in a kind of pincer movement by Gingrich and former US senator Rick Santorum, who lost the Iowa caucuses to Romney earlier this month by a razor-thin margin.

Romney defended his “record of success” in the private sector, arguing that his business and managerial acumen makes him the candidate best able to defeat Obama and revive the sluggish US economy.

Romney and Gingrich traded accusations over negative ads run by multi-million-dollar outside groups, with Romney calling a film about Bain Capital made by Gingrich supporters “the biggest hoax since Big Foot.”

Gingrich — who blamed attacks by Romney supporters for his poor showing in Iowa — mocked Romney’s contention that he had no control over such groups, saying it “makes you wonder how much influence he’d have if he were president.”

As in previous debates, Romney reserved his harshest criticism for Obama, accusing the president of trying to turn the United States into a “European social welfare state.”

Gingrich meanwhile defended his characterization of Obama as “the food stamp president” when a debate moderator questioned whether it was belittling to poor Americans and minorities.

He also stirred up a rare standing ovation from the audience when he defended his idea of employing poor children as public school janitors.

“They’d learn to show up for work. They could do light janitorial duty…. They’d be getting money, which is a good thing if you’re poor. Only the elites despise earning money,” he said.

Texas Governor Rick Perry, meanwhile, in a foreign policy stumble, described Turkey — a key US ally and NATO member — as a state “ruled by what many would perceive to be Islamic terrorists.”

The field in the Republican race has narrowed to five ahead of Saturday’s vote, after Jon Huntsman, the former U.S. ambassador to China, dropped out Monday after running in last place in recent polls.

The most moderate candidate in the race, Huntsman called on Republicans to unite in support of Romney, who has already won the first two 2012 nominating contests in Iowa and New Hampshire.

His departure left hardened social conservatives Santorum, Gingrich and Perry — all hoping the evangelical base in South Carolina will swing in their favor — along with Texas Representative Ron Paul.

The candidates sparred on foreign policy after Paul — a staunch opponent of foreign aid and military intervention — suggested Washington should have worked harder with Pakistan to track down and arrest Al-Qaeda chief Osama bin Laden rather than kill him in a May 2011 commando raid.

“I would say that maybe we ought to consider a golden rule in foreign policy,” he said, to a roar of boos from the audience.

“We endlessly bomb these countries and then we wonder why they get upset with us.”

Romney fired back, saying: “of course you take out our enemies, wherever they are. These people declared war on us. They’ve killed Americans…”

“The right thing for Osama bin Laden was the bullet in the head that he received. That’s the right thing for people who kill American citizens.”

According to a new CNN/ORC poll released Monday, frontrunner Romney and Democrat Obama are in a dead heat, with the Republican taking a slight 48-47 percent edge among registered voters.

Obama came out ahead among all respondents, 49-47 percent, and had the advantage over Gingrich, Santorum and Paul, according to the poll.

Obama’s approval rating slumped to 47 percent, from 49 percent a month before. The poll was carried out on January 11-12 among 1,021 respondents, and has a margin of error of three percentage points.